FBGs and chirped FBGs are widely used technologies to fabricate complex filters. Gain flattening filters for erbium doped fiber amplifiers (EDFAs) are but one example. Gain flatness of optical amplifiers over the communication bandwidth is a key requirement of high performance optical Wavelength Division Multiplexing (WDM) communication systems. Usually, a gain flattening filter with a spectral response matching the inverse gain profile is incorporated within the amplifier to flatten its gain.
Several gain flattening filter technologies can be used to perform the gain equalization, thin film filters and chirped FBGs being the most widely used, as described in “Gain equalization of EDFAs with Bragg gratings”, Phot. Tech. Lett. 11, 536–538 (1999), M. Rochette, M. Guy, S. Larochelle, J. Lauzon, F. Trépanier. A key metric of performance for gain flattening filters is the insertion loss error function (ILEF): the difference between the measured attenuation of the filter and the target spectra. The target spectra is specific to each amplifier design and is closely related to the inverse gain curve. Because amplifiers are often cascaded along a link, the cumulative effect of the error function of the individual filters is also of importance. Individual filter ILEF smaller than or equal to ±0.1 dB for the full operating temperature and wavelength range of a system are often required, and the ILEF must be as random as possible to avoid the additive effect of systematic errors. In the case of thin film filters-gain flattening filter, the manufacturing process is such that all gain flattening filters have very similar error functions of the order of ±0.25 dB and these systematic errors can add up to unacceptable levels.
The chirped fiber Bragg grating is an attractive technology to produce very low error gain flattening filters. Although several manufacturing approaches are possible, gain flattening filters are typically inscribed in photosensitive fibers using UV light and a chirped phase mask to create an interfering pattern with linearly changing period along the grating. The amplitude of the resulting index modulation can also be shaped by controlling the intensity of the UV-light along the phase-mask. This shaping and trimming process at the UV-writing station is required to obtain low ILEF.
UV-induced defects are responsible for the grating formation but these defect sites are not thermodynamically stable and the change in refractive index can be reversed. This is why gratings are then subjected to a stabilization process, which is a controlled temperature anneal. This annealing progressively removes the most unstable defect sites and the final grating is stable within the system tolerances for the intended grating lifetime. Of course, the annealing step reduces the refractive index modulation and consequently, the grating must be written stronger in order to hit the post-annealing target. This manufacturing process is quite adequate for ILEF of the order of ±0.25 dB. However, imperfections in the phase mask, mechanical and laser instabilities make it very difficult to obtain ILEF smaller than ±0.15 dB. In those cases, a lengthy manual UV-trimming process is often required. Even then, the subsequent temperature annealing process can slightly distort the final spectral shape and the resulting production yield is low. Finally, because the UV-trimming process is operator dependent, it often leads to small but noticeable systematic errors in the ILEF. Very similar process steps apply to other types of complex filters based on FBGs and chirped FBGs. In those cases, the metric can be something other than the ILEF but the general method and apparatus of the present invention which will be described thereinafter would apply equally.